This invention relates generally to semiconductor circuits and more particularly to fabrication of features within semiconductors circuits.
As it is known in the art, gallium arsenide is a preferred material for formation of high operating frequency and high speed monolithic integrated circuits. In particular, with so-called monolithic microwave integrated circuits, a gallium arsenide substrate supports an active layer for formation of field effect transistors and other circuit devices including passive components and transmission lines. These circuits may be classified as analog type circuits and, therefore, can encompass the various types of analog circuits including high power circuits such as power amplifiers. For high power circuits an individual heat dissipating device such as a field effect transistor during operation can often have a channel temperature which exceed 200.degree. C. Such a high temperature influences the performance, the properties, and the reliability of the device. With these high temperatures, heat must be efficiently removed from the device in order to minimize the risk of premature failure of the device.
Typically, the backside, that is the side of the gallium arsenide substrate not having formed thereon the active devices, is provided with a ground plane which is typically formed from a highly thermally and electrically conductive material such as gold. Gallium arsenide, however, has a relatively low thermal conductivity when compared to the thermal conductivity of gold. Therefore, it would be desirable from a device heat sinking perspective to provide a relatively thin substrate of gallium arsenide, to thereby better effectively remove heat from gallium arsenide circuits. On the other hand, gallium arsenide is also a relatively fragile and mechanically weak material. That is, from the perspective of handling of the devices manufactured from gallium arsenide, a relatively thick substrate is desirable to increase the mechanical handling capability of circuits formed from the gallium arsenide.
Moreover, since in many analog circuits, the gallium arsenide substrate acts as a dielectric for strip conductor type transmission lines, for example, the thickness of the gallium arsenide substrate is also a consideration from a device properties perspective.
Therefore, considerations other than the effectiveness of removing heat from the heat dissipating elements often dictate the thickness of the gallium arsenide substrate. A solution to this problem has been to selectively thin a region of the substrate underlying the heat dissipating element to providing a region which can be filled with a conductive material and thus, reduce the thermal resistance between the heat dissipating element and the heat sink or backside conductive layer formed on the backside of the gallium arsenide substrate.
Generally, these circuits also employ so-called plated via hole structures which are provided to make electrical contact between the backside of the substrate and components formed or supported on the frontside of the substrate. These via holes are provided completely through the substrate, whereas the so-called tub structures are provided only partially through the substrate. Techniques for forming via holes and tub structures require two separate photolithographic or lithographic steps and two separate etching steps. Problems arise from performing the second lithographic and etching step on the backside of the wafer after the formation of either one of the via hole or tub structures due to the gross topological differences presented by the features already formed on the backside of the substrate. For example, it is often difficult to provide adequate resist step coverage over regions not to be etched. Therefore, errosion of the resist around the edges of these regions provides unwanted etching of the gallium arsenide during subsequent etching steps. Moreover, a technique which reduces the number of processing steps, should reduce the cost and improve the yield of such circuits.